LVDS and LVPECL are industry standards for differential high-speed signal interconnection on printed circuit boards. Each has its own specifications for termination at the receiver.
The Low Voltage Differential Signalling (LVDS) standard is defined in the document ANSI/TIA/EIA-644-A.
Low Voltage Positive Emitter Coupled Logic (LVPECL) is a de facto industry standard developed by Motorola and derived from earlier signaling schemes ECL and PECL.
An integrated circuit receiver device may embed differential signal terminations within its circuitry for the purpose of providing a more highly integrated, flexible and efficient circuit solution to the user.
LVDS is a differential signaling scheme in which each side of the output differential pair alternates as the source and the sink of current to the output. Its operating differential current ranges from 2.5 mA to 4.5 mA, which translates into a differential voltage swing of 250 mV to 450 mV when a transmission line pair is end-terminated differentially with a 100 ohm resistor as shown in FIG. 1. No external path to ground is required to terminate a direct-coupled LVDS signal. The common mode voltage is nominally 1.0V.
The LVPECL output structure is characterized by a pair of bipolar transistor open emitters, which alternately switch between on and off states. In order for current to flow from the emitters to ground, external circuit pathways must be provided. At the receiver, the required differential voltage swing is 1.1V, with a DC bias of Vdd-1.3V.
FIG. 2 shows a typical 3.3V LVPECL interconnection circuit with standard termination. The reference voltage Vtt may be implemented as an actual voltage supply, or as equivalent arrangements of resistors referenced to Vdd and ground.
It will be appreciated that these termination circuits are significantly different, yet it would be desirable to provide an on-chip termination circuit solution, where a single integrated circuit is able to cope with both types of signal.
U.S. Pat. No. 6,362,644 discloses a programmable termination circuit, which requires the application of external bias voltages.